Cannot boot Snow Leopard from external USB WD drive

I have backed up my Snow Leopard from iMac to a 1TB WD My passport Ultra external hard drive using Carbon Copy Cloner.

I can boot into this drive on my iMac running Snow Leopard.

The drive can also be seen on my macbook air with yosemite and files can be accessed. However, I can not boot into Snow Leopard from my macbook air-

the grey login screen just freezes- the macbook air restarts but I can not get past that grey screen. Aafter about 10 mins I just shutdown the macbook air.

The cables are fine, I have run Disk Utility etc. The problem seems to be in the start up process perhaps. Any help would be appreciated. Thanks!